Flat styling of select2.

select2-override.css

.select2-container .select2-choice {
    height: 34px;
    -webkit-box-shadow: none;
    -moz-box-shadow: none;
    box-shadow: none;
    background-color: #fff;
    background-image: none;
    background: #fff;
}

.select2-container .select2-choice .select2-chosen { margin-top: 4px }
.select2-container .select2-choice abbr { top: 13px }
.select2-container .select2-choice,
.select2-container .select2-choice .select2-arrow {
    -webkit-border-radius: 0;
    border-radius: 0;
    border-color: #ccc;
}

.select2-container .select2-choice .select2-arrow {
    background-color: #fafafa;
    background-image: none;
    background: #fafafa;
    width: 22px;
}

.select2-container .select2-choice .select2-arrow b>span { margin: 4px 0 0 6px }
.select2-container.select2-container-active .select2-choice { border-color: #91acce }
.select2-container.select2-dropdown-open .select2-choice { border-bottom-color: #ccc }
.select2-drop {
    -webkit-border-radius: 0 !important;
    border-radius: 0 !important;
}

.select2-drop:not(.select2-drop-above) { margin-top: -3px }
.select2-drop .select2-results { max-height: 300px }
.select2-drop .select2-results li {
    line-height: inherit;
    margin: 0;
    padding: 0;
}

.select2-search { margin: 4px 0 }
.select2-search input,
.select2-search input:focus {
    background-color: #fff;
    background-image: none;
    background: #fff;
    -webkit-box-shadow: none;
    -moz-box-shadow: none;
    box-shadow: none;
    border: none;
}

.select2-container.form-control {
    border: none;
    -webkit-box-shadow: none;
    -moz-box-shadow: none;
    box-shadow: none;
    padding: 0;
}

select2-override.less

// mixins
.border-radius(@radius) {
	-webkit-border-radius: @radius;
	border-radius: @radius;
}

.box-shadow(@shadow) {
	-webkit-box-shadow: @shadow;
	-moz-box-shadow: @shadow;
	box-shadow: @shadow;
}

.background-color(@color) {
	background-color: @color;
	background-image: none;
	background: @color;
}


// closed dropdown
.select2-container {

    .select2-choice {
        height: 34px;
        .box-shadow(none);
        .background-color(white);

        .select2-chosen {
            margin-top: 4px;
        }

        abbr {
            top: 13px;
        }

        &, .select2-arrow {
            .border-radius(0);
            border-color: #ccc;
        }

        .select2-arrow {
            .background-color(#fafafa);
            width: 22px;

            b > span {
                margin: 4px 0 0 6px;
            }
        }
    }

    &.select2-container-active .select2-choice {
        border-color: #91acce;
    }

    &.select2-dropdown-open .select2-choice {
        border-bottom-color: #ccc;
    }
}

// open dropdown
.select2-drop {
    .border-radius(~"0 !important");

    &:not(.select2-drop-above) {
        margin-top: -3px;
    }

    .select2-results {
        max-height: 300px;

        li {
            line-height: inherit;
            margin: 0;
            padding: 0;
        }
    }
}

// search input
.select2-search {
    margin: 4px 0;

    input, input:focus {
        .background-color(white);
        .box-shadow(none);
        border: none;
    }
}

// Bootstrap 3 form-control fix
.select2-container.form-control {
    border: none;
    .box-shadow(none);
    padding: 0;
}

Have worked little bit more to make it FullyFlat design select2. Below is the link for updated code:
https://github.com/techhysahil/Select2-Flat-UI/blob/master/Overide.css